Europe’s building stock is entering a transition phase, becoming an active player in the energy system, controlling, producing, storing, and consuming energy. The explosion of smart technologies enabling a more efficient use of energy in buildings will inevitably redesign the built environment and the linked energy flows. This study attempts to offer a presentation of the legislative framework and targets of EU on energy efficiency and how this policy and long-term strategy affects the national plan of Greece for energy efficiency in building sector. Cases of energy upgrading of residential and office buildings in Greece will be presented and the amount of investment will be assessed in relation to the percentage of energy upgrade. The article concludes framing the five pillars of a smart built environment for EU.